This invention relates generally to the processing of electrostatic images and more particularly is concerned with applying toner particles to such images.
In the field of xerographic or electrostatic image recording, a latent image is formed on a plate or other member. This latent image is the pattern which is projected onto the member by a lens system or the like. Prior to the projection, the member is charged by corona thereby providing a large number of electrons at or near the photoconductive surface which are capable of migrating when struck by photons. The projection or exposure, as it may be termed, causes electrons to leave the surface, in proportion to the amount of impinging light at any increment of the surface. The latent image is thus formed of electrons and the absence of electrons.
The charge originally placed on the photoconductive surface will decay in time and with it the latent image will fade. The image is made visible, according to the art, by the application of toner particles thereto, these particles having electrostatic charges which cause them to adhere to the electrons on the photoconductive surface of the member. In this manner the image is formed on the member. Toner particles are minute particles of carbon, resins and the like.
In most xerographic equipment, the member to which the particles adhere comprises a selenium drum and the drum is pressed against a sheet of paper to transfer the image to the paper. Then the paper surface carrying the toner particles is rapidly heated causing the particles to fuse permanently to the paper. In other apparatus the latent image is formed directly on the sheet of paper which has been coated with a zinc oxide-resin mixture that is photoconductive. This process is known as electrofax, and the toner is applied in liquid form, the liquid normally being a hydrocarbon within which the toner particles will achieve surface charge. The xerographic process using a drum utilizes a type of toning known as cascade toning in which the toner particles are mixed with plastic beads and applied in this dry mixture form. Only the toner particles adhere to the selenium surface of the drum, this being the photoconductive surface.
The invention was developed for use with electrostatic image apparatus of a nature that differs from those mentioned above, primarily in that the construction of the member carrying the latent image is different, giving rise to many advantages. Principally, the speed of the photoconductive surface is so great and its sensitivity so high that the member can be used much in the same manner as the ordinary photographic cameras, but without many of the disadvantages thereof. The member itself comprises a structure that can be called an electrophotographic film, comprising a substrate of polyester or the like carrying an ohmic layer bonded thereto and an inorganic photoconductive coating bonded to the surface of the ohmic layer. Reference may be made to the copending patent application for detailed descriptions of said electrophotographic film.
This electrophotographic film has a very high dark decay characteristic and a large differential between dark and light decay characteristics. Its photoconductive surface can be charged very quickly -- in a matter of a fraction of a second -- and exposed at rates comparable with those used to expose high speed silver halide emulsion photographic films. The primary manner of use of the electrophotographic film of said copending applications is to produce transparencies, since the combination of the ohmic layer and the photoconductive coating on the clear plastic substrate will provide an extremely thin lamination which is transparent to a substantial degree -- between about 70 percent and 85 percent. Accordingly, in forming the visible image, it is desired to apply toner to the photoconductive surface and thereafter to fuse the toner particles directly to the lamination itself.
Due to these desires and the characteristics of the electrophotographic film, it is essential that the toning of the member be done as rapidly as possible after exposure has occurred. This, or course, is to prevent the decay of the latent image along with the decay of the charge on the photoconductive member. The apparatus and method of the invention were designed for and are especially adapted for such utilization, but are not so limited. The teachings of the invention are applicable in other techniques and with other apparatus as well.
Aside from the requirements of the high speed electrophotographic film which has been mentioned above, all of which are met by the invention, many disadvantages of prior methods and apparatus for toning have been obviated. These disadvantages which are mentioned hereinafter are caussed by the nature of the xerographic or electrofax process to some extent, but the methods and apparatus for toning are a considerably factor as well.
Where baths are used, as in electrofax, the images which are produced are inherently nonuniform because the sheet of paper carrying the photoconductive surface is immersed into the bath from one edge and passes progressively through to the other. The decay of the image and surface charge is still going on while this occurs so that there is bound to be fading from the leading to the trailing edge. In the case of the high speed electrophotographic film of the copending applications the fading would be aggravated in a bath-type of toning process. The apparatus and method of the invention provide for application of the toner suspension simultaneously over the entire photoconductive surface of the electrophotographic film.
Another problem with prior methods and apparatus is fidelity or tone, in the popular sense, of the resulting image. Where an exposure is proper with respect to a given set of conditions, some time later, after the bath of toner in liquid has been used for a while, the toner becomes depleted. The concentration of toner particles changes and the image resulting loses contrast and depth.
As will be understood from the description of the invention the method and apparatus call for the toner suspension to be accurately and precisely prepared in advance, encapsulated in a suitable enclosure, used in one instance and the remainder discarded. In this way, total uniformity is achieved every time. Exposure can always be precisely related to the toner concentration by the manufacture and can be relied upon to obtain optimum results for each use. If desired, without any changes whatsoever, the enclosure may be used several times prior to discarding.
In prior apparatus toner application has produced bad grey scales and nonuniform black areas. The xerographic equipment in use at the present time cannot produce images with large uniform dark areas. This is known as edge effect. Such areas come out light in the center and dark along the edges, this being caused by the tendency of the toner particles to seek the maximum field differential thereby migrating to the edges of the images. According to the invention, this is obviated by a novel biasing method and apparatus which will be described, the resulting images being uniform in color and having excellent grey scale characteristics.
Electrostatic image producing apparatus which utilize dry toner application in the processing of the photoconductive surfaces of the members used have problems of uniformity, waste, and difficulties in handling. There are mixtures of plastic beads and iron filings which have to be handled, separated, and the foreign particles and surplussage discarded. Often, where dry foreign particles are included in the mixtures, the application of toner is inefficient because of adherence to the foreign particles. Other structures use magnetic brushes to provide the necessary surface charge to the toner particles, this being more apparatus to handle and keep clean.
Given the necessity of building a small hand-held camera type of device for producing the electrostatic record, prior methods of processing require apparatus which is complex, bulky and expensive. It is unlikely that such a device could be manufactured with the known technology represented by such prior methods and apparatus. The invention herein, on the other hand, makes such a project feasible and practical.
In electrostatic image producing apparatus known, the fields represented by the latent images on the photoconductive surfaces are relatively weak. Furthermore, their strength decreases with distance from the surface by exponential factors so that the attraction for toner particles is not very great. Accordingly, such apparatus depends in some instances importantly on gravity to bring the particles within the stronger portions of the fields close to the surfaces comprising the photoresponsive layers, and in other instances the toner is mechanically agitated to throw the particles into such fields. Two adverse results are nonuniformity and time loss. Surfaces which are not horizontally disposed with the toner applied on the upper side suffer to some greater extent from these two problems.
The bias used in accordance with the invention drives the particles directly at the photoconductive surface and thus obviates the above-described disadvantages. Specifically, besides uniformity, the processing or timing is accomplished substantially instantaneously and irrespective of the disposition of the photoconductive surface.
A large measure of difficulty with prior apparatus is additionally eliminated by providing disposable capsules of toner which are economical and easy to use, besides giving uniform results.
Many advantages which are not specifically described above are inherent in the invention and these will become apparent from the description which follows.